Caracterización geoquímica de las anatexitas del Domo del Tormes (provincias de Salamanca y Zamora)

Abstract

The Tormes Dome is an heterogeneous anatectic area, wich is made up mainly by Namuro-Westphalian granites, together with glandular gneisses, fine-grained leucogneisses and pelitic rocks in different migmatization stages. All the granites are peraluminous and potassic. According to their prevailing autochthonous character, no significant differentiation processes are evident. However, different granitic groups appear to have some distinctive features with genetic implications. Thus, the major group, the leucogranites, shows high SiO2 content, low TiO2 and CaO contents and a high Fe/Fe+Mg and Rb/Sr ratios, with low Ba content. They have low REE contents, and a low or moderately negative Eu anomaly. One of these leucogranites, the fine-grained type, could be an anatectic product derived from the fine-grained leucogneisses, wich are crustal rocks with low REE contents. The porphyritic biotite granitoids show similar characteristics to the so-called "Older Granodiorites". Relative to the leucogranites, they exhibit lower SiO2 and Fe/Fe+Mg values; higher TiO2 and CaO contents; lower Rb/Sr ratio and higher Ba content, and also a REE increased content, with a low or moderately negative Eu anomaly. All of these features are suggestive of an igneous protolith, wich could be related to associated tonalitic rocks of lower crustal or mantle origin, without rejecting a metagray wacke source as another possibility. The porphyritic biotite-sillimanite granites have intermediate character with a negligible negative Eu anomaly and a marked LREE fractionation. These features, together with a moderate Rb/Ba ratio, support a metasedimentary pelitic protolith. Finally, the available biotite analyses are consistent with the distinctive compositions of each granitic group.